1. Field of the Invention
The present invention relates to piezoelectric speaker technology, in particular, a lumped parameter resonator of a piezoelectric speaker.
2. Description of the Related Art
With the advent of multimedia computers in laptop computer systems, audio speaker systems providing high quality sound have been integrated into laptop computers. The surface area of laptop computers, however, has been a limiting factor in providing speakers within a laptop computer. Accordingly, laptop computers, particularly laptops having relatively thin dimensions, have switched from diaphragm-type speakers to piezoelectric speakers.
In piezo speaker technology, a piezoelectric actuator placed on a speaker panel converts electrical energy into mechanical energy, thereby driving the speaker panel to achieve a moment or mode and produce acoustic energy. A piezoelectric actuator is a flat strip or disk of ceramic having crystals which stretch and shrink in a transverse direction in response to electrical signals. The transverse motion of the piezoelectric actuator produces transverse motion by the speaker panel allowing the panel, which is typically a polycarbonate sheet, to serve as a structure born vibration resonator. While the transverse movement of the speaker panel is radiation efficient at high frequencies, the transverse movement is radiation inefficient at low frequencies.
In a conventional portable computer having a piezoelectric speaker, pseudo-translational motion is generated by the ends of a speaker panel in addition to the transverse motion of the speaker panel. As a piezoelectric actuator stretches and shrinks, the ends of the speaker panel flex forward and backward. While the pseudo-translational motion is more efficient than transverse motion at low frequencies, the pseudo-translational motion has been subject to wave cancellation. When a speaker panel is placed in a moment or mode, there is compression of air molecules on one side of the speaker panel and refraction of air molecules on the other side of the speaker panel. As a result, both front waves and back waves are produced. At the unbaffled ends or edges of the speaker panel, the front and back waves destructively interfere with one another resulting in zero delta pressure or difference. Zero delta pressure means that the human ear is unable to hear bass since the human ear detects pressure differences. Thus, a conventional portable computer using a piezoelectric speaker has been inefficient at radiating low frequency sounds such as a kick drum or a deep male voice.
A variety of modifications to a piezoelectric speaker panel have been ineffective in eliminating wave cancellation. For instance, sealing up or baffling the sides of a piezo speaker panel has the drawback of reducing speaker panel excursion. Without panel excursion, low frequencies are not radiated. Another modification has been to increase the size of the speaker panel. While using a larger speaker panel reduces the time necessary for front waves and back waves to cancel, the panel has continued to be radiation inefficient below a certain frequency, albeit a lower frequency than for a smaller panel. Yet another modification has been to increase the number of piezoelectric actuators driving the speaker panel. While increasing the number of piezoelectric actuators creates greater speaker panel displacement, the improved radiation efficiency is achieved merely for high frequencies. Another piezo speaker modification has been to use a speaker panel having a material composition with a high degree of stiffness such as aluminum honeycomb. While using a stiffer piezoelectric speaker panel has marginally increased the radiation efficiency of the piezoelectric speaker, a critical frequency has remained at and below which the piezo speaker is radiation inefficient. In addition, using a stiffer piezo speaker panel has the drawback of reducing the number of modes for the speaker panel. Thus, a contemporary piezoelectric speaker has maintained poor radiation efficiency at low frequencies.
Further, a conventional piezoelectric portable computer has included a plastic lid and a display screen forming a frame structure. The plastic lid has been used as a piezoelectric speaker panel by using the plastic lid as a structure born vibration resonator. The display screen, however, has been hard mounted in plastic and has not been used as a piezoelectric speaker panel.